The present invention relates to wax impregnated filters for use in investment casting apparatus and an improved method for producing the filters.
The investment casting industry represents a significant market for ceramic foam filters due to high metallurgical standards (dye penetrant, magnetic particle, visual and radiographic). In a high percentage of investment casting applications, a filter is installed into a bottom-pour gating system during the wax tree assembly process. This method of gating incorporating the filter results in a quiet and non-turbulent mold filling with minimal splashing of the pouring stream and hence minimal chances for possible reoxidation after the filter. The installation of the filter into a bottom-pour gating system, however, requires a significant amount of costly preparation work to be performed on the filter.
The filter must be impregnated or coated with sprue wax in order to prevent the penetratiOn of dipping slurries into the filter structure during the shell building process. In addition, two blocks of sprue wax typically must be attached to the entrance and exit faces of the filter. These blocks of wax serve three functions: (1) mechanically retaining the filter in the gating system after autoclaving to remove the sprue and pattern wax; (2) providing an expansion in the gating system to accommodate the increased filter area required to maintain metal flow; and (3) providing surfaces to attach the wax filter assembly to the bottom pour gating system.
Three methods of filter impregnation are currently used commercially. The first is a ceramic border technique involving the application of a colloidal silica bonded fused silica border to the edge(s) of the filter body. Following air drying, the body of the filter is manually impregnated with wax. Blocks of wax are then attached to each face of the filter. The filter/wax assembly is thereafter dipped in a sealing wax, taking care not to completely coat the ceramic filter border, in order to fill the re-entrant gap where the filter border and wax blocks meet. The advantage of this method is that the filter border is bonded directly onto the shell during the shell building process. As a result of this, the filter cannot rattle in the shell and possibly chip and introduce filter material into the mold cavity. In addition, there is no gap between the filter structure and the shell wall. Thus, molten metal bypass around the edges of the filter is effectively negated. The disadvantage of this method is that it is very labor intensive and costly.
A second method involves preparing a die cavity into which the pre-cut sized filter is inserted. The filter is impregnated with wax by either pouring or injecting wax into the die cavity. Wax attachment blocks may be incorporated into the die cavity. The major disadvantage is that the filter is usually loose in the shell after wax removal in the autoclave. This is due to variations in the filter dimensions which dictate that the die cavity be oversized. The loose filter is troublesome in that it can rattle and possibly chip causing loose filter material to be introduced into the mold cavity as inclusion material. Because of the gap, molten metal can bypass around the edge of the filter.
The third method involves wrapping the filter with sheet wax, sealing with a hot knife, and attaching two blocks of wax to each face of the filter. The disadvantage of this method is that after autoclaving the filter is loose in the shell due to the gap created when the sheet wax is removed. Once again, the filter can rattle and chip and molten metal can bypass around the edges of the filters through the gap created when the wax sheet is removed. This method of filter encapsulation is also labor intensive and costly.
Accordingly, it is an object of the present invention to provide filters pre-impregnated with sprue wax and ready for installation or use by a foundry in investment casting apparatus.
It is a further object of the present invention to proved an improved method for producing the above filters.
It is still a further object of the present invention to provide a method as above which produces the filters inexpensively.
It is yet a further object of the present invention to provide a method as above which produces filters having improved dimensional tolerances.
These and other objects and advantages will become more apparent from the following description and drawings in which like reference numerals depict like elements.